This invention relates to a heating apparatus comprising a high-frequency generator and at least two inductors connected to the high-frequency generator for inductively heating workpieces in which each inductor is formed by an induction coil comprising a high-permeability coil core, which coil cores can be displaced mutually independently.
Such a heating apparatus is known from the U.S. Pat. No. 3,109,909.
Since high-frequency generators for industrial heating purposes are relatively expensive arrangements, it is generally desired to connect two or more inductors to a single high-frequency generator in such a heating apparatus.
It is likewise desired in this context that these inductors can be switched on and off mutually independently. If they can be switched independently of one another it is then possible to have different workpieces or different parts of a workpiece undergo an individual heat treatment per inductor, requiring the high-frequency generator to be on.
Switching the inductors on and off mutually independently is possible, for example, by switching the current to or the voltage through an inductor. Specifically, with relatively large power level this will cause problems, however, because large currents cause a high amount of dissipation in a conductive switch, and when switching high voltages, sparkover will readily occur.
The heating apparatus in the above U.S. Pat. No. 3,109,909 comprises four inductors connected to a single high-frequency generator, each inductor consisting of a single induction coil and a coil core, the latter being formed by a fixed portion and an adjustable portion. Each workpiece or part of a workpiece receives an individual heat treatment because the inductor can be adapted to the shape of the workpiece with the aid of the adjustably mounted coil core. This adjustability is realised by threadedly adjusting the core or using a different type of rigid positioning. Such rigid positionings do not generally allow the apparatus to be readily modified, as a result of which they are less suitable for use in a heating apparatus that has to operate automatically, as described in the preamble.
Consequently, the known heating apparatus is not suitable for manufacturing processes in which workpieces may have a large variation of form and/or size, leading to a specific process parameter showing an ever different variation in time. Therefore, this heating apparatus is unsuitable for automatically processing such workpieces.